The present invention relates to liquid, storage-stable isocyanates prepared by reacting solid or semi-solid diphenylmethane diisocyanates. The most commercially important diphenylmethane diisocyanates which are solid at room temperature are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof, which melt at 39.degree. C. and 34.5.degree. C., respectively.
Diisocyanates which are liquid at room temperature have obvious advantages compared to those diisocyanates which are fused solids or slurries at ambient temperature. A liquid is easier to pump and less expensive to transport. A liquid has a homogeneous composition as supplied without the need to homogenize it at elevated temperatures as is the case with slurries or fused solids. In the production of polyurethanes, a liquid can be added easily by weight or volume, and combined with suitable coreactants at ambient temperatures. This is safer than using the materials at elevated temperatures due to the lower vapor pressure of the materials at room temperature.
Numerous patents have issued relating to the liquefication of diphenylmethane diisocyanate. See, for example, U.S. Pat. Nos. 3,152,162; 3,384,653; 3,394,165; 3,449,256; 3,640,966; 3,641,093; 3,674,828; 3,701,796; 3,883,571; 4,014,935; 4,055,548; 4,088,665; 4,031,026; 4,102,833; 4,115,429; 4,118,411; 4,154,752; 4,177,205; 4,229,347; 4,261,852; 4,321,333; 4,332,742; 4,490,300; 4,490,301; 4,490,302; 4,539,156; 4,539,157; 4,539,158; and 4,883,909. Generally, these related to processes which chemically modify the diisocyanate, and yield an adduct. A disadvantage common to all of these processes is that they are not reversible to the original diisocyanate.
It is known from U.S. Pat. No. 3,644,457 to react 4,4'- and/or 2,4'-diphenylmethane diisocyanate with a branched aliphatic dihydroxyl compound or polyethers based on 1,2-propylene glycol to produce a product which is liquid at room temperature. In a similar manner, U.S. Pat. No. 4,055,548 teaches that diphenylmethane diisocyanate can be liquefied by reaction with ethylene glycol-based polyethers.
Liquid diphenylmethane diisocyanates have also been produced by reacting diisocyanates having specified 2,4'-isomer contents with propylene and polypropylene glycols and with polyoxyethylene glycols (see e.g. U.S. Pat. Nos. 4,118,411 and 4,115,429).
It has also been proposed to prepare liquid diphenylmethane diisocyanate compositions by reacting the diisocyanates with three separate alkylene glycols, each having at least three carbon atoms (see e.g. U.S. Pat. No. 3,883,571), or by the same reaction wherein at least one of the glycols is dipropylene, tripropylene, or polypropylene glycol (see e.g. U.S. Pat. No. 4,229,347).
U.S. Pat. No. 4,332,742 discloses the preparation of storage-stable, liquid diphenylmethane diisocyanates by reacting the diisocyanate with an N-substituted ethanolamine.
Urea group-containing polyisocyanate mixtures which are liquid at room temperature are also known. See U.S. Pat. No. 4,611,083.
It is also known that diphenylamine diisocyanates and the higher derivatives can form stable prepolymers with primary amine terminated materials having a degree of amination of from 25 to 100%. See U.S. Pat. No. 4,686,242.
Prepolymers are also known which are prepared by reacting amine reacting amine-terminated polyethers with a variety of different isocyanates including diphenylmethane diisocyanates. See U.S. Pat. 4,689,356.
It is an object of this invention to provide diphenylmethane diisocyanates which are storage-stable and liquid at 25.degree. C.